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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old October 6th, 2019, 11:43   #1
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Join Date: Apr 2016
Location: Virginia
Default /// changed tire size ... how to recalculate mileage ///

Hi all,

I went from the factory size tires at 195 65 r 15 to
205 70 r 15.

Based on my GPS my speedometer is now more accurate Speed wise but how does that affect my mileage calculations?

I have read that the factory speedometer reads a little on the fast side relative to real speed and upsizing the tire makes it the speedo correct but I think the odometer was set to read correctly with the factory tire size ..... so now the odoneter is not correct?

so now that I updated my tire size I think I need to use a fudge Factor when calculating my mileage...

I'm not sure how my friend calculated it, but I think my fudge factor is 5%?

Meaning I take my miles and / gallons to get miles per gallon and then multiply that figure by 1.05 to get my corrected miles per gallon does this sound right?

Thanks in advance for any and all help

2003 Jetta tdi wagon , 5 sp, 316k miles, 11 mm pump, DLC 520, Malone stage 1, P205 70 R 15 Ecopias on Avus rims, hammer mod

Last edited by Andyinchville1; October 6th, 2019 at 11:52.
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Old October 6th, 2019, 17:31   #2
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Join Date: Feb 2012
Location: Cowford, FL

I use this website: https://www.rimsntires.com/specspro.jsp and put in my values. RIM SIZE and OFFSET aren't important for this calculation.

If I put 195 65 15 in the old and 205 70 15 in the new, it gives me this information:

OLD: 635 mm (25'')
NEW: 669 mm (26.3'') - 34 mm (5.4%) taller.

OLD: 807.1 revs per mile
NEW: 76631 revs per mile.

This means that your car rotates 5.3% less than it did before to cover the same distance. If your car says you drove 1000 miles, you really drove 1053 miles. Thus, any mileage your car tells you will be inaccurate: you need to multiply what your car says by 1.053. This is exactly what you have, so you should be good!

My VWs have all had a 0-6% lower indication on the speedometer than what I was actually doing, according to my GPS measurements, but the odometer has always been 100% accurate.
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Old October 7th, 2019, 11:22   #3
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My 205’s put the speedo spot on and the odometer 7 miles light per 100. So when calculating I multiply the odometer miles by 1.07, if yours is short 5%, multiply by 1.05
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Old October 8th, 2019, 15:59   #4
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Join Date: Apr 2016
Location: Virginia


Thanks for the input guys, A friend of mine somehow calculated the 5% error factor to increase my hand calculations by ... I wasn't 100% sure on the math involved but I am happy to see at least 2 people came up with a similar fudge factor.

Thanks for the input .... I feel better about my fuel mileage now! ;-)

2003 Jetta tdi wagon , 5 sp, 316k miles, 11 mm pump, DLC 520, Malone stage 1, P205 70 R 15 Ecopias on Avus rims, hammer mod
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